1. Technical Field
The present invention relates to a light-emitting-element driving circuit, and in particular to a light-emitting-element driving circuit which drives a plurality of light-emitting elements.
2. Background Art
In recent years, various electronic devices such as portable phones are equipped with a light-emitting-element driving circuit. For example, Patent Literature 1 (JP 2008-251886 A) discloses a structure comprising a drive current supply circuit which is connected between a first power supply and a second supply and in series with a light-emitting element, and supplies a drive current to the light-emitting element according to a voltage on a control terminal, and a current-determining circuit which determines and outputs a current according to an amount of output light of the light-emitting element. The structure further comprises a current-voltage converter circuit which converts a current determined by the current-determining circuit into a voltage and outputs the converted voltage to the control terminal of the drive current supply circuit when a control signal is in a first state, and which disconnects the output voltage terminal from the control terminal of the drive current supply circuit when the control signal is in a second state. The structure also comprises a reset circuit which connects the control terminal of the drive current supply circuit to the second power supply when the control signal is in the second state.
Among known light-emitting-element driving circuits, there exists a light-emitting-element driving circuit in which a plurality of light-emitting elements are arranged in a matrix form and are sequentially caused to emit light. As shown in FIGS. 3A-3D, in a plurality of light-emitting elements 16, 26, 36, and 46 placed in parallel with each other, switching elements for the light-emitting element 12, 22, 32, and 42 are sequentially switched, and, for example, in order to emit light from only the light-emitting element 16, a common switching element 8 is controlled to be switched ON during a first period in FIG. 3A.
More specifically, during the first period, as shown in FIG. 3A, the switching element for light-emitting element 12 and the common switching element 8 are controlled to be switched ON and the switching elements for light-emitting element 22, 32, and 42 are controlled to be switched OFF, so that only the light-emitting element 16 emits light. Then, during a second period, as shown in FIG. 3B, the common switching element 8 and the switching element for light-emitting element 12 are controlled to be switched OFF, the switching element for light-emitting element 22 adjacent to the switching element for light-emitting element 12 is controlled to be switched ON, and the switching elements for light-emitting element 32 and 42 are maintained in the OFF state.
Then, during a third period, as shown in FIG. 3C, the switching element for light-emitting element 22 is controlled to be switched OFF, the switching element for light-emitting element 32 adjacent to the switching element for light-emitting element 22 is controlled to be switched ON, and the common switching element 8 and the switching elements 12 and 42 for light-emitting element are maintained in the OFF state. Finally, during a fourth period, as shown in FIG. 3D, the switching element for light-emitting element 32 is controlled to be switched OFF, the switching element for light-emitting element 42 adjacent to the switching element for light-emitting element 32 is controlled to be switched ON, and the common switching element 8 and the switching elements for light-emitting element 12 and 22 are maintained in the OFF state. The control then returns to the switching control during the first period shown in FIG. 3A. In this manner, a switching control for switching the states of FIGS. 3A, 3B, 3C, and 3D in this order is repeated periodically.
As shown in FIGS. 3A-3D, parasitic capacitances 14, 24, 34, and 44 are formed at anode terminals of the light-emitting elements 16, 26, 36, and 46, and, when the switching elements for light-emitting element 22, 32, and 42 are controlled to be switched ON, charges are accumulated in the parasitic capacitances 24, 34, and 44. In the first period in which the light-emitting element 16 emits light, when the common switching element 8 is controlled to be switched ON as shown in FIG. 3A, the charges accumulated in the parasitic capacitances 24, 34, and 44 flow as leakage currents, and there is a possibility that light is emitted from the light-emitting elements 26, 36, and 46.